Theanine is known as a principal component of deliciousness of green tea and is an important substance as a flavor component of food such as tea. It is pointed out that γ-glutamyl derivative containing theanine acts as a biologically active substance in animals and plants. Particularly for human, it is known that theanine has effects of suppressing and ameliorating various symptoms accompanying diminished homeostasis such as sensitivity to cold, menopausal symptoms, sleep disorder, autonomic imbalance symptoms, etc. (Patent document 1).
An extremely large amount of tea needs to be taken when theanine is taken only from conventional green tea in order that the above-mentioned physiological effect may be developed. Accordingly, a technique for dispensing high-concentration theanine in beverage has been desired in order that a large amount of theanine may be taken more easily.
On the other hand, when tea beverage is sealed in a container, there is a problem that a macromolecular component such as pectin, hemicellulose or the like separates out as precipitation during long term preservation. Some methods have been reported to solve the problem (Patent documents 2 and 3, for example).
Patent document 2 discloses a technique for filtering green tea extract by an ultrafilter membrane to eliminate a macromolecular component. Although this technique can achieve the prevention of precipitation, flavorful components of tea adsorb to a filter medium. As a result, there is a disadvantage that a flavor particular to tea is diminished.
Furthermore, patent document 3 discloses a technique for preparing green tea in an acid range and thereafter quenching the green tea so that turbidity is enhanced. The turbidity is eliminated by centrifugation, and a supernatant solution is added with diatom earth and is filtrated. Thereafter, pH is re-prepared into a neutral range. Filtration of diatom earth also removes flavor component of tea in this technique.    Patent document 1: JP-A-2000-247878    Patent document 2: JP-A-H04-45744    Patent document 3: JP-A-H04-311348